The present invention relates to a catalytic converter for cleaning the exhaust gas of an internal combustion engine of a car. More specifically, the present invention relates to a catalytic converter for effectively removing nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbons (HC) contained in such exhaust gas.
Conventionally, a so-called three-way catalytic converter has been used for removing the harmful substances (NOx, CO, HC and the like) from exhaust gas. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as platinum, palladium and/or rhodium. This three-way catalytic converter works as a reduction catalyst for reducing NOx to N2 while also working as an oxidation catalyst for oxidizing CO and HC to CO2 and H2O. In this way, the three-way catalytic converter is capable of cleaning exhaust gas by removal of the harmful substances such as NOx, CO and HC.
In recent years, various researches have been made to improve the performance of a three-way catalytic converter. An example of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO2) provided coexistently with a three-way catalyst. CeO2 has an oxygen-storing capacity (OSC). The OSC is the ability to occlude excess oxygen in an oxygen-excessive atmosphere and to release the occluded oxygen in an oxygen-deficient atmosphere. In an oxygen-excessive atmosphere, for example, reducing reaction occurs reluctantly. In the presence of CeO2, it occludes the excess oxygen to reduce the concentration of oxygen. Thus, when exhaust gas having a high oxygen concentration is supplied to the exhaust gas cleaning catalytic converter, CeO2 occludes oxygen to lower the oxygen concentration of the exhaust gas, thereby enabling the three-way catalyst to effectively perform reducing reaction. Conversely, when exhaust gas having a low oxygen concentration is supplied to the exhaust gas cleaning catalytic converter, CeO2 releases oxygen to increase the oxygen concentration of the exhaust gas, thereby enabling the three-way catalyst to effectively perform oxidation.
In cars, there is an increasing demand for dealing with higher restrictions on cold emissions, so that the exhaust gas cleaning catalytic converter is required to effectively clean exhaust gas even at relatively low temperature when the engine is not sufficiently warmed up immediately after its start for example. Thus, the catalytic converter needs to have high catalytic activity at low temperature. For this purpose, the engine tends to be located at the exhaust manifold which is near the engine rather than below the body floor. Due to the location at the manifold, the catalytic converter may be often exposed to high temperature of no less than 900xc2x0 C. (or sometimes no less than 1,000xc2x0 C.). Thus, the catalytic converter needs to provide high durability at such a high temperature.
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which is capable of providing good catalytic activity at low temperature while also providing high resistance against high temperature.
According to the present invention, a catalytic converter for cleaning exhaust gas comprises a first coating layer formed on a heat-resistant support, and a second coating layer formed on the first coating layer. The first coating layer contains alumina which supports palladium. The second coating layer contains Cexe2x80x94Zr complex oxide which coexistently carries platinum and rhodium, and Zrxe2x80x94Ce complex oxide which differs in composition from the Cexe2x80x94Zr complex oxide and which coexistently carries platinum and rhodium.
In a preferred embodiment of the present invention, the Cexe2x80x94Zr complex oxide may be represented by the following general formula.
Ce1xe2x88x92(x+y)ZrxMyO2xe2x88x92zxe2x80x83xe2x80x83(1)
In the formula (1), M represents a rare earth element other than Ce and Zr or an alkaline earth metal. z represents the degree of oxygen deficiency determined by the valence and proportion of the contained element M. Further, 0.25xe2x89xa61xe2x88x92(x+y)xe2x89xa61.0, 0xe2x89xa6xxe2x89xa60.55, and 0xe2x89xa6yxe2x89xa60.2.
The Zrxe2x80x94Ce complex oxide may be represented by the following general formula.
Zrixe2x88x92(a-b)CeaNbO2-cxe2x80x83xe2x80x83(2)
In the formula (2), N represents a rare earth element other than Ce and Zr or an alkaline earth metal. c represents the degree of oxygen deficiency determined by the valence and proportion of the contained element N. Further, 0.55xe2x89xa61xe2x88x92(a+b)xe2x89xa61.0, 0xe2x89xa6axe2x89xa60.45, and 0xe2x89xa6bxe2x89xa60.2.
In a preferred embodiment of the present invention, the second coating layer has a surface layer portion which coexistently carries platinum and rhodium.
In a preferred embodiment of the present invention, the second coating layer has a surface layer portion which carries one of platinum and rhodium alone.
In a preferred embodiment of the present invention, the first coating layer additionally contains barium salt of an inorganic acid.
In a preferred embodiment of the present invention, the first coating layer supports 30xcx9c100 g of alumina and 0.5xcx9c8.0 g of palladium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the Cexe2x80x94Zr complex oxide carries a total amount of 0.3xcx9c3.0 g of platinum and rhodium per liter of the heat-resistant support, whereas the Zrxe2x80x94Ce complex oxide carries a total amount of 1.0xcx9c3.0 g of platinum and rhodium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the surface layer portion of the second coating layer carries a total amount of 0.05xcx9c2.0 g of platinum and rhodium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the first coating layer additionally contains Cexe2x80x94Zr complex oxide which does not carry any precious metal.
In a preferred embodiment of the present invention, the second coating layer additionally contains alumina which does not support any precious metal.